1. Field of the Invention
The present invention relates to a spindle motor employed in a recording disk apparatus such as a hard disk drive unit (HDD), in particular, to a spindle motor comprising a stationary axis fixed to a housing of the recording disk apparatus, and a hub body supported around the stationary axis by a bearing for rotation so as to receive a recording disk.
2. Description of the Related Art
The hub of a spindle motor employed in an HDD is often made of aluminum. One reason is to meet the demand for a lighter weight. Another reason is to allow the linear expansivity of the hub to coincide with that of an aluminum recording disk. The uniform linear expansivity is considered to avoid displacement between the recording disk and the hub, which holds the recording disk, under a temperature change.
Aluminum has a relatively large thermal expansion coefficient, 23.5.times.10.sup.-6 /.degree. C., so that the hub suffers from thermal expansion when the HDD is operated and the temperature rises. The thermal expansion causes the hub to expand in the radial direction, so that the rotation center may be decentered in the recording disk and/or the hub may wobble with respect to an annular outer race of the ball bearing. In particular, in case the annular outer race is made of a material, such as steel, having a thermal expansion coefficient that is relatively smaller than that of aluminum, significant eccentricity and/or wobble are expected. The eccentricity and/or wobble may cause displacement of a magnetic head off a target recording cylinder on the recording disk. Therefore, the recording density for a magnetic disk can hardly be improved unless such eccentricity and wobble are suppressed.